Wet-Bulb Globe Temperature - A more useful definition of heat
Air temperature is the most common indicator of heat, and a very useful one. But we all know from experience that 30°C in the sun on a windless, humid day, feels different from 30°C in the shade with a breeze: the air temperature only tells part of the story. A more comprehensive metric of the potential for heat stress is the Wet-Bulb Globe Temperature, abbreviated to WBGT.
See how it worksWhat is WBGT? The physical meaning
The Wet Bulb Globe Temperature (WBGT) combines air temperature, humidity, wind, solar, and thermal radiation into a single environmental heat indicator. Humidity is important for physiological heat stress, because we can cool ourselves down by sweating and evaporating that sweat, but higher humidity reduces the efficiency of sweat evaporation, and less wind also reduces evaporative cooling as well as convective cooling. Solar and thermal radiation transfer heat directly to the body: we feel the warmth of sunlight, as well as the heat radiated by dark surfaces such as asphalt during and after a hot day.
For outdoor conditions in sunlight, WBGT is defined as:
Air temperature
Air temperature is the temperature we all know, measured in the shade.
Wet-bulb temperature
The wet-bulb temperature literally reflects the temperature an imaginary wet bulb will reach when put outside. It is the air temperature cooled down by the maximum evaporative cooling possible under the prevailing conditions, and as such an indicator of how much you can cool down by sweating. The wet-bulb temperature gets higher as humidity increases and as wind decreases.
Globe temperature
The globe temperature represents the temperature inside a black (absorptive) globe under the prevailing solar and thermal radiation and wind conditions. More radiation increases the globe temperature, whereas wind reduces it through convective cooling. The globe temperature is sensitive to the amount of direct sunlight, affected by clouds, for example, and also to the time of day: at noon when the sun is directly overhead, the solar radiation intensity is highest.
It may seem like the 0.1 coefficient for air temperature is strangely low, but wet-bulb and globe temperature are also inherently tied to air temperature. You could see them as humidity- and radiation-corrected air temperature. As a result, the WBGT is highly correlated with air temperature.
Because of the different components of WBGT, the same WBGT value can occur in different conditions. Figure out how this works below.
WBGT can be measured directly or reconstructed from meteorological data. Physically based approaches such as the Liljegren method derive it from air temperature, humidity, wind, radiation, pressure and solar position. We compute our WBGT values from meteorological conditions along the route at the time of exposure, including the wind generated by the riders themselves.
- Wet bulbHumidity + wind
- 24°C
- Black globeRadiant heat
- 39°C
- AirAmbient temperature
- 31°C
What is WBGT? The physiological meaning
WBGT is internationally established as a measure of environmental heat stress, and has been shown to be the most practically useful environmental indicator to assess physiological heat risks. It is defined in ISO 7243, and used in sports guidance, such as by the International Olympic Committee (IOC) consensus statement on recommendations and regulations for sport events in the heat and the American College of Sports Medicine (ACSM) position statement for recognition, prevention, and management of exertional heat illness. Sports associations use WBGT to specify prevention and adaptation measures, and the International Cycling Union (UCI) has its own heat protocol based on WBGT.
WBGT thresholds and the UCI high temperature protocol
WBGT thresholds are not standardised across different countries or entities. For example, the ACSM WBGT thresholds for athletes differ from the heat protocol of the International Cycling Union (UCI).
The UCI WBGT-based high temperature protocol:
| WBGT (°C) | Risk level and countermeasures |
|---|---|
| < 15°C | Very low risk: No specific countermeasures. |
| 15.0–17.9°C | Low risk: Warm-up in the shade with fans, skin protection with non-greasy sun creams, choice of light-coloured clothing, normal hydration plan. |
| 18.0–22.9°C | Moderate-low risk: Warm-up with ice vests, use of fresh towels, application of strict, individualised hydration plans, distribution of "ice-socks", supply of ice to the teams during the race. |
| 23.0–27.9°C | Moderate-high risk: Adaptation of the start area to keep riders in the shade before the start, protect officials, organising staff and volunteers from the sun, increase the number of neutral motorbikes providing riders with drinks and ice packs, adapt the rules limiting hydration and cooling in competition. |
| Above 28°C | High risk: Modification of start and finish times, possible neutralisation of a section of the race or stage, cancellation of the stage/race. |
The moderate-high risk category starts at a comparatively low WBGT of 23°C, which is appropriate for long-lasting high-strain exercise like Tour de France stages. We use the UCI thresholds in our analyses.
What WBGT does not tell us
The WBGT values we show describe the thermal environment encountered by riders. They do not directly represent an individual’s physiological experience. We do not currently have data or models for core body temperature, heart rate, sweating, dehydration, metabolic heat production, acclimatisation, health status or individual fitness.
This distinction is particularly important for the Tour de France riders, since they generate very large amounts of metabolic heat. Rider WBGT should consequently be interpreted as an indicator of environmental exposure, not as a direct measure of experienced heat or danger to a particular rider. The effect of the activity itself is crucial for understanding heat risk at sporting events, and we aim to expand this work towards indicators that explicitly represent exercise intensity, metabolic heat, acclimatisation and physiological responses.
WBGT for spectators
We show WBGT values experienced by cyclists, which include the cooling they generate through riding wind. For spectators, we compute WBGT values that only depend on ambient wind. Because of the lower wind-induced cooling, spectator WBGT values are slightly higher than rider WBGT values. This (naturally) does not mean that the riders are exposed to lower physiological heat stress than the spectators. Metabolic heat, generated by the body itself through exercise, is not part of the environmental indicator WBGT. This component is of course much larger for riders compared to spectators, and would result in a higher overall heat stress for riders. Nonetheless, Tour de France spectators often stand on the side of the road for multiple hours, on exposed climbs with little shade and strong solar irradiance. Therefore, heat risks for spectators should not be ignored.